Overview
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ABSTRACT
The presence of soft alluvial materials in superficial geological structures modifies observed seismic motion. This amplification (sometimes de-amplification) of seismic motion is due to the velocity contrast between stiff layers and soft alluvial layers. The geometry of the alluvial layers and the surface topography also influence “seismic site effects”. This article describes the physical phenomena that lead to seismic site effects, and details how to quantify and model them for practical engineering applications. Lastly it describes the possibilities for mitigating seismic site effects.
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Read the articleAUTHORS
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Emmanuel JAVELAUD: Doctor of Engineering Science from Tokyo Institute of Technology, Engineer from ENSG-Nancy - EDF engineer - EDF-CEIDRE-TEGG (Aix-en-Provence, France)
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Jean-François SEMBLAT: Doctorate from the École Polytechnique, Ingénieur en Chef des Travaux Publics de l'État - IFSTTAR (Champs-sur-Marne, France)
INTRODUCTION
The presence of loose materials in subsurface geological layers modifies seismic movement. This modification is essentially a function of the nature of the materials making up the surface formations, their geometry and the amplitude of the seismic movement.
Evaluating the response of surface formations to seismic stress is fundamental to earthquake engineering, in order to determine the characteristics of seismic movement at the surface.
Knowledge of surface seismic motion is required for a number of practical applications, such as :
dimensioning foundations under seismic loads ;
assessment of the potential risks of liquefaction under the water table and induced settlement outside the water table;
and structural justification.
Surface seismic motion is also required as input data for soil-structure interaction analyses.
The aim of this article is to present the influence of seismic site effects on free-field seismic motion, in terms of amplitude and frequency. It begins by detailing the phenomena involved, illustrating them with real-life examples. It then describes the behavior of soils when subjected to seismic loads and the geotechnical investigations required to study site effects.
Using simple analytical examples, this article also details the parameters involved in the amplification or damping (de-amplification) of seismic waves.
The consideration of seismic site effects in current building codes is then placed in the context of existing simplified or detailed approaches. This allows us to introduce potential evolutions in future generations of codes.
Finally, this work presents a method for seismic protection of structures by acting on the various components of seismic site effects.
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KEYWORDS
topography | building | shock waves | seismic building | Constructibility | earthquake | geology | lithology
This article is included in
Soil mechanics and geotechnics
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Seismic site effects for surface structures
Bibliography
Software tools
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Cyberquake – This software models the response of soils to seismic shocks.
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Code-Aster – This is a free software package for numerical simulation of structural mechanics.
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Standards and norms
- Calcul des structures pour leur résistance au séisme. Partie 5 : Fondations, ouvrages de soutènements et aspects géotechnique. - NF EN 1998-5 AFNOR - septembre 2005
Regulations
Decree no. 2010-1254 of October 22, 2010 on seismic risk prevention.
Décret n° 2010-1255 du 22 octobre 2010 portant délimitation des zones de sismicité du territoire français.
Arrêté du 22 octobre 2010 relative à la classification et aux règles de constructions parasismiques applicables aux bâtiments de la classe dite "à risque normal" (Order of October 22, 2010 on the classification...
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